What happens to "usable once only" cards when Illusion is played?

Question

In a Munchkin game yesterday, a level 8 player (let's call her "Jennifer") was fighting a Hippogriff. After counting gear bonuses, Jennifer's combat strength was 15 and the Hippogriff's was 16.

If Jennifer won the combat, she would have won the game, so people started unloading cards that affect combat. The following cards were played:

• Jennifer used a "+3 to either side, usable once only" (+3 UOO) card to help herself, making the fight 18/16 in her favor
• Another player used a +3 UOO card to help the Hippogriff, making the fight 18/19.
• Jennifer used a +2 UOO card on herself, and another player used a +2 UOO card for the Hippogriff (20/21)
• Jennifer used a +5 UOO card on herself, and another player used a +5 UOO card for the Hippogriff (25/26)
• Jennifer used a "-5 to monster" card on the Hippogriff, and another player used a +5 UOO card for the Hippogriff (25/26)
• Jennifer used a +2 UOO card on herself, making the combat 27/26 in her favor

At this point, another player threw down Illusion and Plutonium Dragon to replace the Hippogriff. The relevant part of Illusion is

Discard any one monster in this combat, along with any cards that have been played to modify it, and replace with a monster card from your hand.

We then had a long argument about which cards remained in play.

All of us immediately agreed that the "-5 to monster" card was a modifier for the Hippogriff, and disappeared with the Hippogriff.

For UOO cards played to help Jennifer, half of us thought that they were gone, because they had already been "thrown," "fired," "drunk" or otherwise used up. The other half thought that the cards remained in play, because it was still the same combat and Illusion didn't explicitly say that anything should be done about cards played to help the munchkins' side.

Most of us argued that the UOO cards played to help the Hippogriff's side of the combat also went away. Some of us cited the wording of Illusion; others thought they too were used up. A few of us maintained that UOO cards are not technically "modifiers," and therefore should have remained in play.

What should have happened in this combat (assuming no further cards were played)?

Answer 1

I don't have an official ruling to cite, but my interpretation would be that Illusion only removes the monster and any Monster Enhancers specific to that monster, but does not affect any one-shot items or other "±X to either side" cards.

What's confusing here is that, instead of using the standard term "Monster Enhancer" (like, say, the Mate card does), the Illusion card text just says to discard "any cards that have been played to modify [the monster]". However, as far as I can tell, these two terms as effectively synonymous; the rules essentially define a Monster Enhancer as any card played to modify an individual monster:

Monster Enhancers

Certain cards, called monster enhancers, raise or lower the combat strength of individual monsters. (Yes, you can have a negative enhancement.) Monster enhancers may be played by any player during any combat.

All enhancers on a single monster add together. If there are multiple monsters in a combat, the person who plays each enhancer must choose which monster it applies to. Exception: Anything that enhances a monster also enhances its Mate … if Ancient, Enraged, and Mate are played on a single monster, in any order, you are facing an Ancient Enraged monster and its Ancient Enraged Mate. Good luck …

(The Mate card text was changed to explicitly refer to Monster Enhancers in The Great 2010 Changeover; I suspect that the lack of a similar change to the Illusion card may have been an oversight.)

The essential difference between Monster Enhancers and one-shot items is that Monster Enchancers modify a specific monster (and possibly its Mate, per the explicit exception quoted above), whereas one-shot items add a bonus to a side in combat.

In particular, if there are multiple monsters involved in a combat (e.g. due to Wandering Monster cards being played), a one-shot item granting a bonus to the monsters' side will not be tied to any specific monster, and will not disappear even if some of the monsters are subsequently removed from combat by any means.

Of course, if all the monsters leave combat with no replacement (and no new wandering monsters appear within the rules-prescribed 2.6 seconds), then the combat is over and any bonuses of any kind become irrelevant.